Currently, digitization of terrestrial broadcasting is in practical use. An OFDM transmission system has been adopted as a terrestrial digital television broadcasting method in Japan and Europe. The OFDM transmission system in Japanese terrestrial digital television broadcasting has been established as a standard. The standard is disclosed in lines 2 to 9, page 8, under Chapter 2: Overview of the ISDB-T System in the standard specification “Transmission System for Digital Terrestrial Television Broadcasting” (ARIB STD-B31 version 1.1) by Association of Radio Industries and Businesses.
A description is made for the conventional technology related to the present invention. An OFDM transmission system modulates and demodulates a signal by allocating the data to plural carriers (carrier waves) orthogonal to each other. Each carrier is modulated by a modulation method such as QPSK (Quadrature Phase Shift Keying), QAM (Quadrature Amplitude Modulation), or DQPSK (Differential Quadrature Phase Shift Keying).
If a synchronous modulation method such as QPSK or QAM mentioned above is adopted as a modulation method for carriers, pilot signals are inserted in the transmission signal. Pilot signals are inserted at regular intervals frequencywise and timewise. The transmission line characteristic is calculated using pilot signals when demodulating the OFDM signal. The transmission line characteristic calculated from pilot signals is interpolated frequencywise and timewise to derive a reference signal for all the carriers. Then, a transmission data series is obtained from the differences in amplitude and phase. Further, the data series demodulated undergoes error-correction coding.
Consequently, digital broadcasting provides favorable reception quality as compared to the conventional analog television broadcasting.
However, the signal level of a specific carrier can drop under the influence of a reflected wave called a multipath, depending on a receiving environment of the broadcast waves. If disturbing signals such as analog television broadcast waves exist in the same frequency band, the demodulation performance can deteriorate under the influence of the disturbing signals. The signal level can drop due to a strong multipath interference, or an interference signal can exist at the frequency position to which a pilot signal has been inserted. In this case in particular, the transmission line characteristic to be a reference for demodulation may be estimated wrongly, and thus the demodulation performance can largely decrease as compared to a case free from a multipath interference and disturbing signals.
Therefore, a method is known that detects an interference affecting a transmission signal, calculates a weighting amount corresponding to an interference amount detected, and performs soft decision by weighting the demodulated signal, to improve error correcting capability of the error correcting part. This conventional makeup is disclosed in pages 37 and 38, under 2.4.3 Maximum Likelihood Demodulation on White Gaussian Channel, in Chapter 2: Fundamental Concept of Code Theory, Code Theory, compiled by The Institute of Electronics, Information and Communication Engineers, written by Hideki Imai (in Japanese).
As a method of detecting disturbing signals, a method is known that makes hard decision for the signal demodulated, calculates the difference between the signal demodulated and that after hard decision, and uses the information on the dispersion of each carrier obtained by integrating the difference value timewise. The above-mentioned conventional makeup is disclosed in Japanese Patent Unexamined Publication No. 2001-44965, for example.
Another method is known that calculates an error between the average amplitude of pilot signals and an amplitude of each pilot signal, and detects presence or absence of an interference conforming to the frequency of a pilot signal. If an interference exists, a pilot signal affected by an interference is not used, but a value obtained by interpolating those free from an interference, prior to and subsequent to the affected one, is used instead. The above-mentioned conventional makeup is disclosed in Japanese Patent Unexamined Publication No. H11-252040, for example.
As aforementioned, the conventional disturbing signal detecting device and OFDM receiver detect a frequency-selective interference by information on dispersion of OFDM carriers, to use for error correction. Alternatively, the devices replace pilot a signal affected by disturbing signals with a signal obtained by interpolating those not affected, according to information on an error in the pilot signal. With these methods, an attempt is made to prevent disturbing signals from deteriorating reception performance.
However, as in the makeup described in Japanese Patent Unexamined Publication No. 2001-44965, which detects disturbing signals from the difference between signals demodulated and those after hard decision, the difference can be calculated between a signal with its mapping point different from the transmission point and the signal demodulated. Meanwhile, the makeup involves an averaging process for data obtained in a certain period of time in order to alleviate the influence of wrong estimation, and thus calculating the instantaneous amount of disturbing signals can be difficult.
In the makeup described in Japanese Patent Unexamined Publication No. H11-252040, which detects an error between the average amplitude of pilot signals and an amplitude of each pilot signal, instantaneously calculating the precise amount of disturbing signals can be difficult.